1. Field of the Invention
This invention relates to electronic imaging devices, and in particular to a CMOS imager cell incorporating a “pinned transfer gate”.
2. Related Art
Electronic imaging devices (“imagers”) find use in a broad range of applications in many distinct fields of technology including the consumer, industrial, medical, defense and scientific fields. Imagers use an array of photoreceptors to convert photons bearing image information into electrical signals representative of the image.
In recent years, CMOS imagers have become a practical implementation option and provide cost and power advantages over other technologies such as charge coupled devices (CCD). A conventional CMOS imager is typically structured as an array of imager cells, each of which includes a photoreceptor approximately reset to a known potential in preparation for integration and readout of an image. The performance of a CMOS imager depends heavily on the performance of the individual imager cells.
In the past, the imager cells took the form of either passive photoreceptor cells, active photoreceptor cells, or transfer gate active photoreceptor cells. The passive photoreceptor cells typically included a photodiode for collecting photocharge and a single access transistor to connect the photodiode to a readout bus. However, passive photoreceptor cells, while having high quantum efficiency, were plagued with high read noise. As a result, imagers began to incorporate active photoreceptor cells. The active photoreceptor cells included a photoreceptor, and either three or four support transistors. The support transistors included a reset transistor, source follower transistor (for buffering and amplifying the collector photocharge), and an access transistor for connecting the photoreceptor to a readout bus. In transfer gate active photoreceptor cells, a fourth transfer gate transistor was used to transfer photocharge from the photoreceptor to a sense node, thereby allowing correlated double sampling, and a corresponding decrease in read and dark current noise.
Active photoreceptor cells, however, exposed far less photoreceptor area to incident light due to the overlying support transistor structures. Furthermore, the n+ contacts used in active photoreceptor cells generated significant dark current, thereby undesirably altering images during integration and readout. In addition, prior photoreceptor cells were not tailored to provide adequate response over a wide range of light levels, nor to blue light in particular.
A need exists for an improved imager cell that addresses the problems noted above and other previously experienced.